Abstract: A deck post (or brace) having a non-circular cross-section is secured to a roof deck. A lead pipe jack (sleeve) is slid over the post. A waterproofing assembly having a collar and a skirt is provided; the collar has an opening with generally the same non-circular cross-section as that of the post. The collar opening is positioned on the deck post, and the unit is slid down the post until the collar is at the top of the sleeve, the skirt extending down over the sleeve. A clamp is then positioned and tightened around the collar to provide a watertight seal on the deck post. When the post is already installed on a roof deck and the sleeve and the waterproofing assembly cannot be slid into place on the post, a modified retrofit waterproofing assembly and sleeve are used to provide the waterproofing. The modified sleeve is a split lead flashing jack with an open seam on one side. The sleeve is opened up, wrapped around the post and soldered closed.